This invention relates to communication systems and more particularly to such systems having multiprocessing capability.
It is desirable for a communication system, such as a Private Branch Exchange (PBX), to grow in size in small increments as the needs of the PBX user increases. Traditionally, PBX's are designed to be optimally economic around a particular size and when the size is surpassed a PBX having a greater processing capacity is usually required.
Some PBX users, when faced with a PBX having limited processing capacity, try to solve the problem by connecting two or more PBX's together. This solution is usually unsatisfactory since the user then perceives a difference of features depending upon which PBX is invoked.
Other problems arise when a user attempts to connect PBX's together in parallel, thereby allowing either PBX to serve any user.
These problems arise because traditional PBX control elements are programmed to process any received stimulus (such as an off-hook signal) to completion before the next stimulus is processed. This serial processing of stimuli ensures that the processing of stimuli keeps in step with the arrival of stimuli. Thus, the control element can be viewed as a batch processor of stimuli that arrive from equipments that are connected to the PBX switch.
A dual PBX arrangement is essentially a second switch and a second control element (module) connected together switch to switch and control element to control element to form switch and control element pairs or a pair of modules. The additional processing power comes from the fact that there are two control elements which now can process stimuli in parallel. This, however, is where the fundamental problem lies.
When two control elements process stimuli in parallel, the general rules of processing stimuli in the order of their arrival, and of processing stimuli to completion results in excess waiting time, and hence, reduced apparent processing capability, since the stimuli for one call must be completely processed before stimuli for a second call can be processed. This is so since both processors interact to complete the called connection, and this one processor spends time waiting for the other processor to complete a processing task with respect to the stimulus which is being processed to completion.